Ionic microanalyzer wherein secondary ions are emitted from a sample surface upon bombardment by neutral atoms



Dec. 10, 1968 R. CASTAING ETAL 3,415,985

7 IONIC MICROANALYZER WHEREIN SECONDARY IONS ARE EMITTED FROM A SAMPLE SURFACE UPON BOMBARDMENT BY NEUTRAL ATOMS Original Filed NOV. 27, 1963 --IMAGE DERIVING MEANS ZZ //////////////7 //fl// ///////H Barman/b 64.: 74/445- 6504655 5'4 002/4 ITTOK/VE Y5 United States Patent 3,415,985 IONIC MICROANALYZER WHEREIN SECONDARY IONS ARE EMITTED FROM A SAMPLE SURFACE UPON BOMBARDMENT BY NEUTRAL ATOMS Raymond Castaing and Georges Slodzian, Paris, France, assignors to Centre National de .la Recherche Scientifique and CFS-Compagnie Generale de Telegraphic Sans Fil, a corporation of France Continuation of application Ser. No. 326,565, Nov. 27, 1963. This application Nov. 23, 1966, Ser. No. 596,731 3 Claims. (Cl. 250--49.5)

ABSTRACT OF THE DISCLOSURE In an ionic microanalyser, Le, a device for forming images of the distribution of atoms of various kinds over the surface of a sample, by means of secondary ions of a predetermined sign emitted by said surface in response to a bombardment, neutral atoms are used for this bombardment. Preferably an auxiliary ion beam is used for creating a layer of particles, whose charge is of the same predetermined sign, in the vicinity of, and parallel to, the surface of the sample.

This application is a continuation application continuing the application Ser. No. 326,565, filed Nov. 27, 1963, and now abandoned for Improvements in Microanalysers.

The present invention relates to microanalyser techniques.

It is known in such techniques to use an ion gun emitting positive ions for bombarding a sample. The positive ions eject from the surface of the sample secondary ions which are taken up by a particle optics system, suitably designed to produced an image of that surface.

By suitably filtering the various components of the image in a known manner, a nap indicating the distribution, on the surface of the bombarded sample, of the elements or isotopes building up the sample may be obtained.

This well known technique has the drawback of not being easily applicable to insulator samples. Under the impact of the primary beam, such samples become charged to a high positive potential, the number of secondary ions being less than the number of primary ions received.

The target then repels the incident beam. As a result, even if some primary ions manage to reach the sample, the initial potential of the secondary ions is ill defined. It varies from one point of the target to another depending on the local electrical conductivity of the sample and the local secondary ion emission coefiicient.

It is an object of the invention to provide a microanalyser of the type described free of this drawback.

A microanalyser according to the invention includes means for bombarding the sample to be analysed by a beam of electrically neutral atoms, which extract from the sample secondary ions, in the same way as charged particles do.

According to one preferred embodiment of the invention, means are also provided for creating on the surface of the sample a layer of positive ions moving in a direction parallel to the surface of the target at as slow a speed as possible. The ions of this layer are selectively attracted by the negatively charged areas of the sample, to the exclusion of the others, and remove their negative charges.

The invention will be best understood from the following description and the attached drawings, in which:

FIG. 1 shows the basic principle of an arrangement according to the invention; and

3,415,985 Patented Dec. 10, 1968 FIG. 2 shows a detail of the arrangement of FIG. 1.

In FIG. 1, a sample 1 which may be of an insulating material is bombarded by means of gun 2.

Gun 2 includes an ion source 20 which emits positive ions, for example argon ions, an accelerating diaphragm 21 and a focusing lens 22. An enclosure 23, filled with a gas at low pressure, is placed in the path of the beam. This gas may be, for example, of the same kind as that used in the ion source. The pressure will be of the order of a few thousandths of millimetre of mercury. The length of the enclosure is a few centimetres. This enclosure is provided with an input aperture 231 and an output aperture 232. Ions entering the enclosure tear ofl? electrons from the gas atoms, without appreciable deviation of their trajectory, and are thus neutralized.

FIG. 2 shows an arrangement which may be used for eliminating residual ions. It includes a deflector 30 and a trap 31 at the outlet of enclosure 23.

As an example, the ions are accelerated at 10 kv.; the sample is at a potential of +3,500 v. with respect to ground. The electrostatic field between sample 1 and the grounded electrode 24, which belongs to the particle opti cal system for the secondary ions, has no action on the neutral atoms passing through electrode 24 to arrive at high speed on target 1 from which they extract positive IOIlS.

By means of the particle optical system 24-25-26, these positive ions are accelerated and focused, thus forming an image of the surface of the sample which is analysed as known in the art and briefly as recalled hereinabove. To this end the focused beam is applied to a complex sys tem, as described for example in French Patent 1,240,658, and represented by means of block 40 in FIG. 1.

Following this bombardment by neutral atoms, and on account of the secondary emission of positive ions, target 1, if it is an insulator sample, takes up local areas of lowered potential, which are not uniformly distributed on the surface of the sample. If these areasbecome too great, they are, of course, liable to cause local variations of the emission potential of secondary ions, whose energy is thus subjected to considerable dispersion. It should be noted that those negative charges are rather small and are substantially reduced, in absolute value, by the use of bombardment by neutral atoms. The invention provides in addition means for further reducing this charge.

A gun 3 is provided for this purpose. This gun also emits positive ions. The angle the beam of these latter ions makes with the direction normal to the target surface should be as small as it is allowed by practical design considerations. Gun 3 includes a source 33, an accelerating diaphragm 34 and a condenser 32. The ions pass through electrode 24, and their energy is adjusted in such a way that they arrive at target 1 with a velocity whose component normal to the target is substantially zero, in the absence of charging up efiects on the target, that is to say when the whole surface of the target is at the uniform positive potential produced by the high voltage pp y They thus form a local layer of positive ions moving in a direction parallel to the target surface which neutralizes negative charges on the sample surface, if there are any present thereon.

The system according to the invention has the following advantages:

(a) The energy of the particles used for the bombardment can be adjusted at will to secure best efficiency in secondary ion emission, without any limitation on the low energy side by considerations of penetration in the extraction field.

(b) The charge accumulated by an insulating sample is much less than in the case of a bombardment of the same density by primary positive ions, for this negative charge is due to the secondary emission only.

The addition of a gun providing auxiliary positive ions provides automatic neutralization of these negative charges.

Of course, the invention is not limited to the embodiments shown and described which were given solely by way of example. For instance, it is known that negative secondary ions may be used for producing the image of the sample, in the place of the positive ones. Using for the primary bombardment a beam of electrically neutral atoms makes it possible to pass very quickly from the operation with positive secondary ions to the operation with negative secondary ions, by simply modifying the various potentials. The neutralizing auxiliary beam must of course be replaced by an electron beam when operating with negative secondary ions.

What is claimed is:

1. A microanalyzer for analyzing the surface of a sample, said microanalyser comprising: a source of primary ions; means for accelerating said primary ions and focusing said primary ions into a primary ion beam di rected toward said sample surface along a predetermined path; means, located in said path, for neutralizing said primary ions, said neutralizing means yielding a beam comprising neutral atoms and non-neutralized ions; means for deflecting, from said path said non-neutralized ions, thus leaving along said path a beam of fast neutral atoms to bombard said sample surface; means for focusing the secondary ions, emitted by said sample surface in response to said bombardment and specific of said sample surface, into a secondary ion beam carrying an image of said sample surface along a further path not infringing upon said predetermined path; and means for selectively deriving from component ions included in said secondary ion beam an image of the distribution of said component ions over said sample surface.

2. A microanalyzer as claimed in claim 1, wherein said neutralizing means comprise an enclosure filled with a gas at a low pressure and having an inlet and an outlet, and wherein said deflecting means comprise a deflector located in said predetermined path between said neutralizing means and said sample, said deflecting means further comprising an ion trap in the vicinity of said defiector.

3. A microanalyzer as claimed in claim 1, further comprising further means for directing a further beam of positive ions toward said sample along a further path, said further means including accelerating means adjusted for causing the ions of said further beam to arrive in the vicinity of said sample with a velocity whose component normal to said surface of said sample is substantially zero.

References Cited UNITED STATES PATENTS 2,890,342 6/1959 Columbe 25049.5

2,976,413 3/1961 Robinson 250--41.9

3,136,908 6/1964 Weintnan 250-41.9 X

FOREIGN PATENTS 1,240,658 8/ 1960 France.

WILLIAM F. LINDQUIST, Primary Examiner.

U.S. Cl. X.R. 250--4l.9 

